1. Field of the Invention
The present invention relates generally to a method for releasing a stuck drill string, and more particularly, but not by way of limitation, to an improved method for releasing a drill string that is stuck due to differential pressure.
2. Brief Description of Related Art
The drilling of oil and gas wells by rotary techniques involves the circulation of a drilling fluid through a drill string. The drill string or drill stem is made up of a plurality of joints of pipe connected to one another. A drill bit is connected to the end of the joints of pipe for drilling a well bore in the earth. A problem sometimes encountered while drilling a well bore is that the drill string will become stuck whereby the drill string is unable to be moved up and down through the well bore. Some of the reasons for the drill string getting stuck include foreign objects in the hole, key-seating, and sloughing formations. However, a situation known as pressure differential sticking is, for most drilling organizations, the greatest drilling problem worldwide in terms of time and financial cost.
Pressure differential sticking occurs when the pressure differential between the column of drilling fluid and a permeable formation exerts a considerable force against the drill pipe and literally pins the drill string to the bore wall. That is, the hydrostatic pressure of the column of drilling fluid exerts a greater force on the pipe than the force exerted on the pipe by the formation pressure thereby holding the drill pipe against the bore wall.
Various techniques have been previously employed to attempt to get differentially stuck pipe free. These techniques includes decreasing the pressure differential between the well bore and the formation, placing a spotting fluid next to the stuck zone for the purposes of trying to breakup the mud cake around the drill string, and applying a shock force just above the stick point by mechanical jarring, or a combination of all the above.
When decreasing the pressure differential, it has long been the practice to decrease the hydrostatic pressure of the mud column by replacing the drilling fluid with a less dense fluid thereby allowing for less pressure differential to exist between the bore hole and formation. A problem that may be encountered with this technique is that to decrease the pressure in the well bore sufficiently to cause the drill string to be released may not allow formation pressures to be adequately controlled whereby formation fluids enter the well bore and migrate to the surface.
Other methods of decreasing the pressure differential between the well bore and the formation have been proposed. These methods involve forming perforations in the drill string at the point where the drill string is stuck. Fluid is then injected down the drill string and out the perforations in an attempt to remove debris and equalize the pressure between the well bore and the formation by injecting fluid into the formation. In theory these methods would appear to be effective, but in practice they have met with little success. The number and size of the perforations formed in the drill string do not allow for a sufficient volume of fluid to be injected into the formation to achieve the desired goal.
Spotting fluids are designed to cause the filter cake to crack and shrink thereby reducing the adhesive forces of the filter cake. The spotting fluid further lubricates the area between the pipe and borehole resulting in less friction and quicker release. More often than not, an extensive period of time is necessary for this to occur which results in an expensive loss of rig time.
To this end, a need exists for an improved method of releasing a drill string that is differentially stuck. It is to such an improved method that the present invention is directed.